Funny enough, the situation has now become, among Muslims, we do not tolerate a person of one sect in the mosque of other sect.
This is, as it sounds, a worldwide phenomenon. I believe that it has existed since time immemorial and that the intention to unite the majority of people under one faith only meant to counteract sectarianism in this way by creating a universally valid order.
From wherever people believe that order has been given to them, the resulting set of rules has been preserved to maintain the multiplicity of believers under this order. It is safe to say that violence has always been used to achieve this. Not only violence against people of other outlandish faiths, but also violence within their own people or against those who endeavoured to change the code of order and to found their own sect.
In every culture, sub-sects have separated themselves from the main structure by adopting parts of the established rules and rejecting other parts. However, as long as they did not discard the actual belief in a supreme authority, they were tolerated and integrated into the main structure after initial struggles and certainly also fatal battles.
What happened here in Europe apparently didn't happen in your part of the world (?), which is known here as the Age of Enlightenment. Which of course means that belief in God was completely questioned and labelled as ‘superstitious’.
Incidentally, apparently also the idea of the Christians themselves, who considered the pagans to be superstitious.
Since we have been dealing with the tendency of a decreasing belief in God for about three hundred years, but nothing is able to fill the resulting void, we are experiencing the phenomenon of what you have described well with your own phenomenon "one sect permits no entrance to another sect".
We get strange cult constructs that make the achievements attributed to the Enlightenment look like a curse in retrospect. Everything becomes a cult: science itself, the subjective existence of the individual, the academic world and especially its faculties of sociology and anthropology, for example.
Human individual existence loses its stability by being confronted with too many facts, which makes navigation through life almost impossible if you are not having the self discipline to know yourself well. Which is said to happen through faith in God and through practising the religious teachings. The assertion of those who said that people would be able to lead a rational life without God comes now under a lot of fire by so far not very active Christians and is seen as a too high of a risk after threehundred years of applied science and progress in technology. Here in Germany, I do not observe that so much but certainly in the US this can be observed. Since everything sooner or later arrives from the states here in Germany - and even earlier in the UK.
Nietzsche, one of our most famous philosophers (both, loved and hated) , is said to have predicted that if God is declared dead, the result will be mass murder. I don't know whether he really predicted it but it is imaginable. The Nazis and their Eugenics are known as anti Christ and worshipping the Roman model of rule, which means as much as "All Romans are under Roman protection, everyone not being a Roman, can be killed any time. No consequences to be feared."
Now, that so far shows that human beings kill other human beings, be they religious or non religious.
Still, the set of rules I inherited from being a Christian, are valid for me. We are Christians and often wouldn't even describe ourselves as Christians. Like a fish in the water, who is not aware of the water.
I will respond to what you else said, later. This is already a long comment. Thank you so much for providing me with information from your so foreign for me part of earth.